When materials with different properties are joined at an atomic level, new characteristics can be obtained, which may never be obtained with each of the materials alone. For example, combination of a p-type semiconductor and an n-type semiconductor brings about properties, such as the rectification performance, the photovoltaic effect, and the electroluminescence, and is therefore widely used in devices, such as diodes and transistors. In addition, heterostructures of magnetic thin films exhibit the tunneling magnetoresistance effect, the giant magnetoresistance effect, and the like, and hence heterojunction occupies a very important position as one of the means for controlling of physical properties.
In recent years, applications of unique properties developed by reducing the material dimensions to nanometer scale has been in very remarkable progress, and importance of heterojunction is pointed out also in the field of nanomaterials. Gold has been considered to be the most inert metal, but it has been revealed that nanometer-sized gold particles supported on an oxide, such as titanium oxide or cerium oxide, show industrially important catalytic reactions, such as the oxidation of CO, the water gas shift reaction, and the selective oxidation of, for example, propylene. From recent studies on the mechanism of the catalytic activities, it has been pointed out that the state of heterojunction between gold nanoparticles and the oxide is indispensable. Regarding methods of forming gold nanoparticle catalysts, generally use is made of a co-precipitation method, a deposition-precipitation method, and the like. In these methods, gold is precipitated on submicrometer-sized oxide powder crystals that have been prepared in advance, and by firing the resultant gold-precipitated crystals at high temperatures, strong bonding between the gold nanoparticles and the oxide powder crystal surfaces is formed.
For example, Non-Patent Literatures (1) to (5) and Patent Literatures (1) to (4) describe gold-oxide composite nanoparticles obtained by liquid phase synthesis, and Patent Literature (5) describes composite nanoparticles of a noble metal and a sulfide.
Specifically, Patent Literature (3) describes production of dumbbell-shaped or flower-shaped nanoparticles, having the first part formed from any one of PbS, CdSe, CdS, ZnS, Au, Ag, Pd and Pt, and the second part formed from any one of Au, Ag, Pd, Pt, Fe, Co and Ni, based on a mixture of nanoparticles containing a hydrophobic outer coating, and a precursor thereof, which can be applied to biomedicals, nanodevices, and the like. However, in respect to the combination between noble metals and oxides, Au—Fe2O3 and Ag—Fe3O4 are only described in examples of Patent Literature (3).
Patent Literature (4) describes the following combined nanoparticles applicable for composite catalysts for use in oxygen electrodes for fuel cells. One is the dumbbell-shaped composite nanoparticles in which one noble metal nanoparticle (having an average particle size of less than 10 nm) is epitaxially joined to one ferrite particle (having an average particle size of 5 nm to 50 nm), and the other is the flower-shaped composite nanoparticles in which two or more noble metal nanoparticles are epitaxially joined to one ferrite particle. Those combined nanoparticles are produced by steps of: heating a mixed solution of a surfactant and an organic solvent, to which a metal oxide precursor and noble metal nanoparticles are contained, into reflux; and precipitating target composite nanoparticles. The ferrite particles contain at least a ferrite of chemical formula: A2+B3+2O4 (wherein A2+ represents an ion selected from the group consisting of Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Mg2+, Zn2+ and Cd2+; and B3+ represents an ion selected from the group consisting of Fe3+, Cr3+ and Mn3+). The noble metal nanoparticles contain at least one element selected from the group consisting of at least Pt, Pd and Ag. However, only Pt—Fe3O4 is described in examples of Patent Literature (4).
Patent Literature (5) describes acorn-shaped binary metal nanoparticles that are anisotropically phase-separated, which are produced by reducing two types of metal salts in polyol at a high temperature, in the presence of a thiol compound, and in which one of the metals is Fe, Co, Ni or Cu, while the other metal is Pd, Pt, Au or Ag. Patent Literature (5) describes that the binary metal nanoparticles are expected to be applied in the fields of magnetic chemistry and catalytic chemistry. However, only binary nanoparticles formed from a sulfide of Co and a sulfide of Pd are described in examples of Patent Literature (5), and no descriptions are found on nanoparticles containing pure noble metal regions, nanoparticles containing base metal oxides, or methods for the production thereof.
Non-Patent Literatures (1) to (5) describe dumbbell-shaped nanoparticles of Au—Fe3O4, Au—ZnO and Au—MnO produced by liquid phase synthesis; however, no descriptions are found on using base elements other than Fe, Zn and Mn.
Furthermore, in Patent Literatures (1) to (4) and Non-Patent Literatures (1) to (3), only the use or a possibility of use of Fe, Co, Ni, Mn, Cu, Mg, Zn, Cd and Cr as the elements of the oxide is described, and no description is given on the use of Sn, Ti, Al, Zr, Ce, Y, La, Si and Ge. Furthermore, even in regard to the elements of the oxide for which a possibility of use is mentioned, no disclosure is found on compounds to be used in the case where those elements are employed. Therefore, it cannot be said that any inventions utilizing those metals are described to the extent that those having ordinary skill in the art can easily carry out such inventions. Furthermore, since such gold-oxide composite nanoparticles formed by liquid phase synthesis essentially contain various impurities, such as ions and organic materials, in the case of using the composite nanoparticles as a catalyst or the like, it is indispensable to carry out washing of poisoning ions, or removal by firing or cleaning of nanoparticle-protecting organic materials. Furthermore, it is not guaranteed that these cleaning steps can be always carried out stably and completely at an atomic level, with good reproducibility. Also, even from the viewpoint that hazardous materials, such as metal carbonyls, are used as raw materials, or from the viewpoint that various elements for the oxides other than those used as described above cannot be selected in a simple manner, there are many problems to be solved in the formation of gold-oxide composite nanoparticles by the liquid phase synthesis.
Patent Literatures (6) to (10) propose methods for forming plural noble metal nanoparticles on the surface of base metal oxide particles, by forming noble metal-base metal nanoparticles (alloy nanoparticles) in an inert gas using an arc melting method, and then, subjecting the nanoparticles to an oxidation treatment. The temperature and time period for the oxidation treatment are defined to be a gradual oxidation treatment at room temperature, or set to 2 minutes to 4 hours at 200° C. to 600° C., or the like. However, in these methods, the alloy nanoparticles have already aggregated, and thus the materials obtainable are in the form that noble metals are precipitated irregularly and non-uniformly on base metal oxide aggregates. Therefore, by those methods, composite nanoparticles, in which one noble metal nanoparticle is combined to the surface of one base metal oxide nanoparticle, cannot be obtained in an independently dispersed state.
Furthermore, in Patent Literatures (6) to (8), formation of noble metal-oxide composite nanoparticles is also carried out, by evaporating a raw material alloy in an inert gas containing oxygen. In those methods, among the noble metal atoms and base metal atoms that have evaporated by heating of the raw material alloy, only the base metal atoms are brought into reaction with oxygen, and associates of base metal atoms and oxygen are formed beforehand. Then, many of the noble metal atoms and the associates of base metal atoms and oxygen coalesce in a gas phase, thereby to grow noble metal-base metal oxide composite nanoparticles. It is reported that the particles formed via such a growing are in the form in which plural noble metal nanoparticles adhere onto one base metal oxide particle. Therefore, according to those methods, composite nanoparticles in which one noble metal nanoparticle is combined to the surface of one base metal oxide nanoparticle cannot be obtained in an independently dispersed state.
Patent Literature (9) describes a method of producing composite ultrafine particles, including, for example: heating and melting raw materials of T·M (wherein T represents Ti, Al or the like; and M represents Au, Pd or the like) in an atmosphere containing at least one gas selected from the group consisting of hydrogen gas, nitrogen gas and inert gases, to form ultrafine particles; collecting the ultrafine particles with a filter; and then heating the collected ultrafine particles in an atmosphere containing oxygen. In this production method, the ultrafine particles collected with a filter are subjected to an oxidative heat treatment in a state in which the ultrafine particles are joined or aggregated with each other, but the ultrafine particles are not subjected to the oxidative heat treatment during floating and in an independently dispersed state. For that reason, this method can only give particulate composite materials, in which a large number of fine particles formed from noble metals, such as Au and Pd, or/and compounds thereof, are precipitated and dispersed on ultrafine particles formed from metal oxides of Ti, Al and the like. Therefore, by this production method, composite nanoparticles in which one noble metal nanoparticle is combined to the surface of one base metal oxide nanoparticle, cannot be obtained in an independently dispersed state.
In contrast, the inventors of the present invention produced, as previously reported in Non-Patent Literature (6), composite nanoparticles composed of a metal region and a copper oxide region, by forming Cu-46 at. % Au alloy nanoparticles in helium gas by the same process as the inert gas evaporation method as described in Examples of the present invention, and then carrying out a high temperature oxidation treatment in a gas phase. However, since the content of Au in the alloy nanoparticles is too high, the alloy nanoparticles cannot be completely oxidized, and the metal region of the composite nanoparticles is in the state of an Au-17 at. % Cu alloy, so that the complete separation of Au was not possible to be realized.